To calculate box fill, follow these steps: Determine Box Size: Identify the volume of the electrical box, which is usually specified by the manufacturer. Count Conductors: Include all conductors entering and leaving the box. This includes hot, neutral, and ground wires. Summary: The National Electrical Code explains the Maximum Number of Wires that can be installed into a box, otherwise known as Box Fill. Accurate box fill calculations are essential for ensuring compliance with electrical codes and for preventing overheating or fire hazards. Box. Wires in the junction box depend on the box size, wire gauge, and code rules. For example, a 4×4 inch box often holds up to 10 wires if you use 14-gauge conductors. You calculate box volume per 314.